1. Field of the Invention
This invention relates to illumination devices comprising light emitting diode (LED) chains and, more particularly, to illumination devices and methods for avoiding an over-power or over-current condition. Specifically, illumination devices and methods are provided herein for determining a target lumens value that can be safely produced by all LED chains at a present operating temperature to achieve a target chromaticity setting without exceeding a maximum safe current level or a maximum safe power level associated with one or more power converters included within the illumination device at the present operating temperature.
2. Description of the Relevant Art
The following descriptions and examples are provided as background only and are intended to reveal information that is believed to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the following information constitutes prior art impacting the patentable character of the subject matter claimed herein.
Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as incandescent and fluorescent light bulbs, including low power consumption, long lifetime, no hazardous materials, and additional specific advantages for different applications. When used for general illumination, LEDs provide the opportunity to adjust the color (e.g., from white, to blue, to green, etc.) or the color temperature (e.g., from “warm white” to “cool white”) to produce different lighting effects.
In some cases, a number of differently colored emission LED chains may be combined into a single package, or emitter module, to provide a multi-colored LED illumination device. A multi-colored LED illumination device may be described as comprising two or more different colors of LED chains combined within an emitter module, typically to produce white or near-white light. Some multi-colored illumination devices may comprise only one emitter module, whereas others may include a plurality of emitter modules arranged, e.g., in a line or an array. There are many different types of white light illumination devices on the market, some of which combine red, green and blue (RGB) LED chains, red, green, blue and yellow (RGBY) LED chains, phosphor-converted white and red (WR) LED chains, RGBW LED chains, etc. within a single emitter module. By combining different colors of LED chains within the same emitter module, and driving the differently colored LED chains with different drive currents, these illumination devices may be configured to generate white or near-white light within a wide gamut of color set points or correlated color temperatures (CCTs) ranging from “warm white” (e.g., roughly 2600K-3700K), to “neutral white” (e.g., 3700K-5000K) to “cool white” (e.g., 5000K-8300K).
Some multi-colored LED illumination devices enable the brightness and/or color of the illumination to be changed to a particular set point. For example, some prior art illumination devices allow the target chromaticity or color set point to be changed by altering the ratio of drive currents supplied to the individual LED chains. As known in the art, the target chromaticity may be changed by adjusting the drive current levels (in current dimming) or duty cycle (in PWM dimming) supplied to one or more of the emission LED chains. For example, an illumination device comprising RGB LED chains may be configured to produce a “warmer” white light by increasing the drive current supplied to the red LEDs and decreasing the drive currents supplied to the blue and/or green LEDs.
Some prior art illumination devices also provide dimming capabilities, i.e., the ability to change the brightness level or target lumens output from the emission LEDs, in addition to (or instead of) color tuning. In most cases, the brightness level may be changed by adjusting the drive current levels (in current dimming) or the duty cycle of the drive currents (in PWM dimming) supplied to all emission LED chains to produce a new target lumens output. For example, the drive currents supplied to all emission LED chains may be increased to increase the target lumens output of the illumination device.
When the drive current supplied to a given LED is adjusted to change the brightness level or color set point of the illumination device, the junction temperature of that LED is inherently affected. As expected, higher drive currents result in higher junction temperatures, while lower drive currents result in lower junction temperatures. Below a certain junction temperature (e.g., about 25° C.), the lumen output of a given LED is generally unaffected by temperature. Beyond this temperature, however, the lumen output of an LED decreases significantly with increasing junction temperatures, thereby requiring higher drive currents to maintain the target lumens and target chromaticity settings of the illumination device. In some cases, the drive currents needed to maintain a certain target lumens and/or target chromaticity setting at a particular operating temperature may exceed a maximum current or power level, which can be safely provided by the power converters, which are included within the illumination device for supplying power to the LED chains.
As the brightness level and target chromaticity settings change, the power delivered to each LED chain by the power converters changes. At certain brightness levels and target chromaticity settings, the power drawn by the combined load (i.e., all LED chains combined) may exceed a maximum safe current or power level attributed to the power converters. This may cause the transformer core of one or more of the power converters to saturate, over-heat and possibly fail, unless counteractive measures are taken.
Some prior art illumination devices include power control circuitry for regulating LED power consumption or for protecting the LEDs from an over-voltage condition. For example, some devices may use current/voltage sensing and feedback to adjust the amount of power supplied to the LED chains by the power converter, and may use voltage clamps to protect the LEDs from electrical damage when the output voltage of the power converter exceeds a maximum value. However, the power control circuitry used in these devices does not protect the power converter from excessive current or power draws when the LEDs are operated at or near maximum operating levels.
A need remains for improved illumination devices and methods for limiting the amount of power drawn from the power converters of the illumination device, so as not to exceed a maximum safe current level or a maximum safe power level when brightness and/or target chromaticity settings are changed.